Geology Reference
In-Depth Information
caused by sub-aerial erosion. These changes in
environment have resulted in an often rapid vari-
aion of rock types in both horizontal and verical
directions. The sediments include sandstones,
limestones, marls, shales and mudstones with a
complete range of transiional types such as cal-
careous sandstones and shaley limestones. These
lithological variaions are too small to be mapped
so that the rocks are classified on the basis of age.
Greater detail is shown on Tanzanian Geological
Survey 1:125 000 maps, where these eist.
Sedimentological evidence indicates that some
present-day deep sea areas were land during the
Neogene (2-2.5 myr
BP),
while areas now land
were deep sedimentary basins of the same age
(Kajoto, 1982). These changes of level resulted
from vertical movements on reacivated Karroo
faults. The offshore islands of Zanzibar, Pemba
and Mafia are thought to be fault blocks (Kent,
Hunt
&
Johnstone, 1971). The inland fault blocks
of, for example, the Ulugurus and Usambaras
were also blocked out along reactivated Karroo
faults, another indicaion of the continental scale
of the movements responsible for the fragmen-
taion of Gondwanaland.
While marine and non-marine sediments were
accumulaing along the coast east of the Mozam-
bique Belt in the newly forming Indian Ocean,
inland, terrestrial conditions prevailed and
sedimentaion occurred in low lying areas such as
crustal depressions, rift valley floors created by
the Karroo fauling and in river valleys.
Figure
2.4.
Regions affected by Pan-African orogenesis dis-
inguished from older cratons and younger fold belts (from
Clifford, 1970).
which has been developing in the Indian Ocean
ever since it was born with the breakup of Gond-
wanaland. This basin extends offshore beyond the
islands of Zanzibar and Pemba for at least 150 km
(Kajato, 1982).
Sedimentation has coninued in this basin since
he Jurassic, although the environmental condi-
tions have oscillated between marine and ter-
restrial, with occasional gaps in the sequence
x
x
x
x
x
x
CRYSTALLIN
E
\
•
OF MOZAMBIQUE BELT
x
\
x
x
x
x
x
K
4 ?
•
\
\ •
? 0
0
x
x
x
x
x
•
,· �·
Figure
2.5.
Westen edge of Somali-Kenya-Tanzania sedimentary basin. Faultline con-
cealed by mid-Jurassic marine sediments which ransgressed on to Basement (based on
Kent
et al.,
1971).
x
x
E
W-
FAULT WITH THROW
3000 - 6000 m
+
Search WWH ::
Custom Search